The present invention relates to high-purity lanthanum, a sputtering target comprising high-purity lanthanum, and a metal gate film mainly comprising high-purity lanthanum.
Lanthanum (La) is one of the rare-earth elements, and is a mineral resource that is contained in the earth's crust as a mixed composite oxide. Rare-earth elements are so called because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust. Lanthanum is a white metal having an atomic number of 57 and an atomic weight of 138.9, and comprises a double hexagonal close-packed structure at normal temperature. Lanthanum has a melting point of 921° C., boiling point of 3500° C., and density of 6.15 g/cm3, its surface is oxidized in the atmosphere, and it gradually melts in water. Lanthanum is soluble in hot water and acid. Although it is not ductile, it is slightly malleable. Lanthanum's resistivity is 5.70×10−6 Ωcm, and it becomes oxide (La2O3) when burned at 445° C. or higher (refer to Dictionary of Physics and Chemistry).
With rare-earth elements, it is generally said that compounds with the oxidation number 3 are stable, and lanthanum is also trivalent. In recent years, lanthanum is a metal that is attracting attention as an electronic material such as a metal gate material or a high-permittivity (High-k) material, and research and development is being promoted.
Lanthanum metal is a material in which high purification is difficult to achieve since it is easily oxidized during the refining process, and a high-purity product thereof did not exist to date. In addition, if lanthanum metal is left in the atmosphere, there is a problem in that the handling thereof is difficult since it will become oxidized in a short time and tarnished to a black color.
In recent years, thinning of a gate insulator film in the next-generation MOSFET is being demanded, but with the SiO2 that has been conventionally used as the gate insulator film, the leak current increases due to the tunnel effect, and normal operation is becoming difficult.
Thus, as a substitute for the SiO2 described above, HfO2, ZrO2, Al2O3 and La2O3 with high permittivity, high thermal stability, and high energy barrier against the holes and electrons in the silicon have been proposed. In particular, among the foregoing materials, La2O3 is valued highly, and a research on its electrical properties and for its use as a gate insulator film in the next-generation MOSFET has been reported (refer to Non Patent Document 1). Nevertheless, with Non Patent Document 1, the subject of research is a La2O3 film, and it does not make any reference to the properties and behavior of La element.
It could be said that lanthanum (lanthanum oxide) is still in the research phase. When studying the properties of such lanthanum (lanthanum oxide), if lanthanum metal itself exists as a sputtering target material, it is possible to form a lanthanum thin film on a substrate and it will be easy to study the behavior at the interface with the silicon substrate and the properties of a high-permittivity gate insulator film or the like by forming a lanthanum compound. There is also a significant advantage in that the freedom of the target as a product will increase.
Nevertheless, even if a lanthanum sputtering target is prepared, as described above, it becomes oxidized in a short time (approximately 10 minutes) in the atmosphere. When an oxide film is formed on the target, the electrical conductivity will deteriorate and thereby cause defective sputtering. In addition, if the lanthanum sputtering target is left in the atmosphere for a long time, it reacts with the moisture in the air and becomes covered with white hydroxide powder, and it may even cause a problem of not allowing normal sputtering to be performed. Thus, after the target is prepared, it is necessary to take antioxidant measures such as by immediately performing vacuum packing or coating the target with fats or oils, but these are extremely complicated processes. In light of the foregoing problems, the actual condition is that a target material based on a lanthanum element has not yet been put into practical application.    [Non Patent Document 1] Written by Eisuke Tokunaga and two others, “Research on Oxide Material for High-k Gate Insulator Film”, The Institute of Electrical Engineers of Japan, Research Paper of Electronic Materials, Vol. 6-13, Pages 37 to 41, Published on Sep. 21, 2001